Show warning on mac when custom pip package path is not writable #768

See #767 (comment)

On Mac OS, you have to add in the general settings that your terminal is allowed to perform updates. Else it can not execute the pip installs. That is interesting.

Maybe we should add a warning if:

PermissionError: [Errno 1] Operation not permitted: '/Users/max/blender/blender-3.3.0-macos-x64/Blender.app/Contents/Resources/custom-python-packages'